The use of pressurized gas to assist in a conventional plastic injection molding process is believed to have been first made commercially practicable by the invention of Friederich disclosed in U.S. Pat. No. 4,101,617 issued July 18, 1978. The Friederich patent addressed the problem of molding hollow shaped bodies in a single injection molding operation, and taught a practicable method of introducing compressed gas along with, or just after, the injection of molten plastic resin into the article-defining cavity. Moreover, the Friederich patent solved the concern of de-pressurizing or relieving the molded article by nozzle separation. The early work of Friederich was directed to the molding of such utilitarian articles as clear plastic architectural bricks and the like. More recently, the patented Friederich process has been adapted to the molding of hollow plastic articles of various shapes and dimensions.
In its early years, the use of pressurized gas in assistance to a conventional plastic injection mold process was not recognized for all of the functional attributes which it is known to enjoy today. More specifically, during those early years, the industry gave greater focus to the use of structural foam as a specialty process for molding relatively thick-sectioned articles which would be light in weight and have acceptable surface finish, i.e., avoid sink marks associated with the conventional plastic injection molding. The range of potential applications of structural foam molding of thermoplastic material was limited, however, due to certain inherent features of such process. Among such features included, the relatively long cycle times required to cool the plastic in the mold (the foam cells serve to insulate heat transfer), and the problem of surface finish (splay, blister and swirl) associated with the foamed, molten plastic resin contacting the cool surface walls of the article-defining cavity.
In recent years, attention has returned to the use of gas assistance with conventional plastic injection molding to attain the product quality and productivity which had been hoped for With structural foam molding. The features of surface quality, lower clamp tonnage, rapid cycle times, weight reduction, material saving and minimization of part distortion or warpage can all be obtained with proper utilization of gas assistance with a conventional plastic injection molding process. The paper titled "GAS-ASSISTED INJECTION MOLDING--THE NEW THERMOPLASTIC MOLDING TECHNOLOGY FOR EXTERIOR BODY PANELS" by Dr. Ken C. Rusch, presented at the 1989 meeting of the Society of Automotive Engineers on Mar. 2, 1989, discusses in greater detail the relevant history of the use of gasassistance in connection with plastic injection molding. Another paper titled "THE AIR MOULD PROCESS--A GAS-ASSISTED INJECTION MOLDING PROCESS" by Mr. Helmut Eckardt, presented at the 1990 Structural Plastics Conference on Apr. 1-3, 1990 provides an updated history.
The impetus for the present invention was the inventor's assignment to realize the successful plastic injection molding of automobile handles which is a relatively thick molding (i.e. has a relatively thick cross section). The making of such a handle required the removal of substantial volume of plastic from the desired part and wherein the injection pressures are relatively low.
There were several practical problems facing the inventor in realizing the handle design in a hollow plastic molded piece. For example, the exterior surface of the door handle had to be "Class A" quality. Any surface degradation due to hesitation marks, blemishes or other imperfections, were unacceptable for commercial standards. Such hesitation marks typically are formed when substantially all or all of the plastic required for the part is injected into the article-defining mold cavity and only then is a gas charge injected into the mold to form the hollow plastic part. The hesitation marks are formed where the flow of plastic stops and then starts again within the article-defining cavity.
One approach to solve this problem is discussed in the U.S. Pat. No. 4,935,191 to Baxi wherein gas is introduced into the molten stream of plastic material immediately after the molten material has passed the position at which the gas is introduced. Such simultaneous injection is also discussed in UK Patent to Hendry et al GB 2158002A.
The primary problem with this approach is that it is difficult, if not impossible, to control the pressurized gas so that it does not blow clear out of the plastic rather than producing the desired gas bubble within the plastic. One reason for this is that the injection pressure of the molten plastic fluctuates during injection. Also, when a gate is employed in the mold, the pressure of the plastic is substantially higher in front of the gate than in the cavity on the opposite side of the gate (i.e. in the article-defining cavity).